This invention is directed towards an improved oxidation and corrosion resistant, low cost, iron-base alloy range which forms an eye-appealing, protective dark oxide coating, is highly compatible with high speed autogenous welding practice, and is particularly suitable for use as electric heater element sheathing.
Electric heater elements currently available usually comprise a resistance conductor enclosed in a tubular metal sheath with the resistance conductor embedded in and supported in spaced relation to the sheath by a densely compacted layer of refractory, heat-conducting, electrically insulating material. The resistance conductor may be a helically wound wire member and the refractory material may be granular magnesium oxide.
The material used for the heater sheath must be low-cost, have excellent resistance to oxidation at elevated temperatures, e.g. 850.degree.-900.degree. C., have resistance to stress corrosion cracking, and exhibit good weldability. In addition, it has now become an important requirement that the material used for the heater sheath possess a desirable appearance. Since electric heater elements are usually exposed and are often present in household items such as range tops and dish washers, consumers prefer that the heater element have an eye-pleasing color, such as black or dark gray.
Presently, a large percentage of heater element sheaths are made from INCOLOY.RTM. alloy 840 (INCOLOY is a trademark of the Inco family of companies). This alloy, disclosed in U.S. Pat. No. 3,719,308, possesses all the necessary properties for use as heater element sheaths. Additionally, its surface oxidizes to a dark gray color. However, the high cost of this alloy, due in large part to its nominal nickel content of about 20%, has prompted a search for a more economical substitute.
Possible lower-cost alternatives are being contemplated, but they all suffer from drawbacks which make them less than ideal. Type 309 stainless steel and Nippon Yakin's NAS H-22 form undesirable greenish oxides. While Type 321 stainless steel oxidizes to a black color and Type 304 oxidizes to dark gray, they are two-phase alloys, and therefore lack adequate strength, and under certain circumstances, can be difficult to autogenously weld.
It is thus an object of the present invention to provide a material to be used as heater element sheathing which exhibits excellent resistance to oxidation at elevated temperatures, and good weldability characteristics through the formation of a critical amount of .delta.-ferrite upon solidification, as defined by a ferrite number of 1 to 15.
It is an additional object of the present invention to provide a heater element sheathing material which forms an eye-pleasing dark gray or black surface oxide layer.
It is a still further object of the present invention to provide a heater element sheathing at low cost.